The present invention relates to a laser plasma x-ray generation apparatus used for an x-ray reduction projection lithography apparatus, an apparatus for highly sensitively analyzing the chemical state, the chemical composition, and the concentration of an impurity, especially a light element, of various types of materials such as semiconductor materials, and the like.
A laser x-ray generation apparatus used in the application purpose described above generally is constituted to irradiate a target with x-rays from a laser plasma x-ray source, which is arranged at a remote position, through a reflecting mirror.
In this case, to improve the resolution, an x-ray source with a short wavelength is used. When, however, the wavelength is equal to or shorter than several hundred xc3x85, the reflectance of the reflecting mirror is extremely lowered and practically becomes zero.
For example, as illustrated in FIG. 1, two kinds of substance layers 2 and 3 are reciprocally and regularly layered with constant thickness on a substrate 1 made of a material such as quartz or silicon, a direct incidence reflecting mirror (multilayer reflecting mirror) with high reflectance utilizing Bragg diffraction can be actualized.
Especially, since as high as sixty and some % of direct incidence reflectance of a soft x-ray with approximately 130 xc3x85 wavelength can be obtained using a Mo/Si multilayer film produced by regularly and reciprocally layering at least several ten Mo layers and Si layers with independently several nm thickness, such a multilayer film has widely been used for x-ray reduction exposure and for a reflecting mirror of an x-ray microscope, an astronomical telescope and the likes.
In this case, synchrotron (SR) radiation light, a laser plasma x-ray (LPX), and a gas pinch plasma (GPP) are employed as an x-ray source and among them, the synchrotron radiation light requires a large-scale and costly apparatus using an accelerator and the gas pinch plasma is a light source with luminance low by some figures, 50 that the laser plasma x-ray, which is a relatively compact and has luminance as high as 1012 w/cm2, is expected to be a practically usable light source.
Nevertheless, as illustrated in FIG. 2, a target material and a fine particle are evaporated by using the laser x-ray source excited by pulsed laser light, so that if the evaporated material and particle adhere to an x-ray optical system, of which the x-ray reflecting mirror is representative, the function of the system is deteriorated.
In order to inhibit such deterioration, a mechanically synchronized shutter or a buffer gas is inserted between the laser x-ray source and the reflecting mirror or the reflecting mirror is positioned at a remote position for converging generated x-rays and using the converged x-rays.
However, in any case, since a reflecting mirror has to be positioned at a position far from a laser plasma x-ray source, which is a point source, the reflecting mirror has a disadvantageous point of a low use coefficient of x-rays and its practical application has been hindered.
Note that, in FIG. 2, 21 shows an incidence laser, 22 shows a target, 23 shows the reflected rays which are created when the incidence laser 21 is reflected by the target 22, 24 shows a carbon thin film-like filter, 25 shows a reflecting mirror, 25a shows a reflecting surface, and 26 shows debris.
FIGS. 3A and 3B independently illustrate a constitution example of a conventional laser plasma x-ray generation apparatus, and represent a paraboloid of revolution and an ellipsoid of revolution used as light concentration reflecting mirrors, respectively. Since a reflecting mirror with a surface of revolution is used in these constitutions, a condensing solid angle used for condensing x-rays from a laser plasma x-ray source serving as a point source is larger than that in FIG. 2. However, laser light is incident on the reflecting mirror along the rotation axis of the reflecting mirror, so that, as will be described later, relatively intense x-rays irradiated in a direction of a normal (vertical direction) to a target material run outside through an aperture which is made on the rotation axis for laser incidence. Alternately, even if the x-rays are reflected by the rotating reflecting mirror, a target material 32 with a finite size reabsorbs the x-rays. Therefore, this poses a problem of low light concentration efficiency.
On the other hand, it has been required to use a soft x-ray with wavelength shorter than approximately 130 xc3x85 in order to further improve the resolution in those application cases. Nevertheless, in the case a Mo/Si multilayer film is used at wavelength 124 xc3x85 (L absorption edge of Si) or shorter, the direct incidence reflectance is considerably lowered on the order of several to about ten % to make it impossible to practically use the multilayer film at the wavelength.
A reflecting surface using Mo in combination with B4C has been proposed in order to increase the reflectance of the reflecting surface 25a, and even such a surface can provide reflectance 30% at the highest and has been unsuitable for practical use.
Recently, the multilayer structure type reflecting surface 25a comprising reciprocally layered Mo and Be layers has been proposed and it is found that higher than 60% of direct incidence reflectance of wavelength just at absorption edge of Be (just at 111 xc3x85) can be obtained by the multilayer film type x-ray reflecting mirror and resolution improvement is expected to be achieved by obtaining high reflectance even of a soft x-ray with a short wavelength in the foregoing application.
Also recently, a cryotarget is invented (Patent No. 2614457) using a chemically inert rare gas element in a liquid or solid state at a low temperature or in a low temperature gas state at vapor density near that in liquid state as a target material irradiated with laser light, thereby eliminating adhesion of the target particle to an x-ray optical system such as a reflecting mirror or others. It can be said that practical application of laser plasma x-ray source (LPX) is just started.
Normally, since a plurality of multilayer reflecting mirrors are employed in an x-ray image-forming optical system as illustrated in FIG. 1, the x-ray concentration and transmittance is generally considerably lowered. For that, each reflecting mirror is required to have a reflectance higher even by a little.
In the case of Mo/Be combination, when a film is actually formed, the interfaces or respective layers become rather rough since Mo and Be are aggregated and owing to the effect of the roughness, there occurs a problem that the actual reflectance is lower than the theoretical reflectance of the multilayer film with an ideal structure by about 20%.
Further, in the case of Mo/Be combination, since the melting point of Be is as low as about 1270xc2x0 C., the reflectance is sharply decreased by use of soft x-rays with high luminance and by increase of the ambient temperature to be high just like the case of the Mo/Si multilayer film type x-ray reflecting mirror and there also occurs a problem of insufficient heat resistance for stable use. Moreover, since the reflectance is extremely lowered at 111 xc3x85 or shorter wavelength in the case of a multilayer film x-ray reflecting mirror using Mo and Be in combination, there occurs another problem that the reflecting mirror can not be usable at the wavelength or shorter.
On the other hand, it was found by Mochizuki, one of inventors of the present invention, that a Xe (xenon) cryotarget laser plasma x-ray source, which is a typical cryotarget, has the most intense emission spectrum near 108 xc3x85 wavelength (A. Shimoura, S. Amano, S. Miyamoto, and T. Mochizuki, xe2x80x9cX-ray generation in cryogenic targets irradiated by 1 xcexcm pulse laserxe2x80x9d, Applied Physics Letters 72, PP. 164-166 (1998)). So far, there exits no reflecting mirror capable of providing sufficiently high reflectance (50% or higher) at the wavelength. Furthermore, though a reflecting mirror having a long life with stable and high reflectance even if being exposed to scattered laser light from intense x-rays and plasma has been required in order to effectively utilize the utmost advantage of using cryotarget, that is, the advantageous characteristic that a reflecting mirror can be positioned in the periphery of a radiation source since adhesion of the target particle can be avoided and that a light concentration optical system having a three-dimensional and wide light concentrating angle can be constituted, any conventional reflecting mirror can not satisfy such requirements.
A main purpose of the present invention is, therefore, to provide a practical laser plasma x-ray generation apparatus with high luminance which uses a light concentration optical system having high light concentration efficiency.
Another purpose of the present invention is, in addition to the foregoing purpose, to provide a laser plasma x-ray generation apparatus which can increase heat resistance further than that in any conventional laser plasma x-ray generation apparatus.
Further, another purpose of the present invention is to provide a laser plasma x-ray generation apparatus using a reflecting surface capable of suppressing reflectance alteration during the use.
Moreover, another purpose of the present invention is to provide a laser plasma x-ray generation apparatus with a longer life than that of any conventional laser plasma x-ray generation apparatus.
In order to achieve the foregoing purposes, the basic concepts of the present invention are characterized in that
(1) Ru or Rh is used as a material for one type of layers and Be is used for the other type of layers, and
(2) a material containing a light element such as B, C, O, N and the likes added to a compound of metals or a single metal as a material for one type of layers.
The means was derived from that the reflectance was improved by 4% when a multilayer film was produced by using Rh instead of Mo in a Mo/Be multilayer film in order to improve the reflectance and then the reflectance of the film was measured at 115 xc3x85. That implied the possibility of existence of a material with a more suitable optical constant than that of Mo. It is, therefore, possible to heighten the x-ray reflectance by using Ru, which is a material with an optical constant more suitable than that of Mo at about the wavelength and further by using a Mo alloy with Ru or Rh and a Ru and Rh alloy instead of Mo.
In addition to that, though the reflectance greatly depends on the optical constant, the actual reflectance tends to be decreased as compared with a theoretical value in the case the roughness of the boundary faces of a multilayer film becomes high. On the other hand, an alloy containing an element such as B, C, O, and N is generally easy to be an amorphous layer when the alloy is formed into an extremely thin layer with a thickness several nm to several tens nm, and being amorphous, the layer of the alloy has smoother boundary faces than a monolayer of an alloy and a metal. Consequently, in the case such an alloy containing those elements instead of Mo, the effect for heightening the reflectance can be taken. For that, in the case a multilayer film comprising such a material used for one type of layers and Be for the other type of layers is employed (1) for application to various analysis utilizing x-rays and soft x-rays, the sensitivity and the precision can be improved owing to the improvement of the reflectance of the multilayer film as compared with that of a multilayer film using Mo layers for layers other than Be layers and in the case of employing the multilayer film (2) for application to x-ray lithography, the throughput can be improved as compared with a multilayer film using Mo for layers other than Be layers owing to the same reason as that for the case of (1).
Moreover, in general, a material containing other elements such as B, C, O, and N added to or compounded with metals including Be frequently has a higher melting point than a single metal and interlayer diffusion can thus be suppressed. Therefore, if such materials are used for constituent materials for the multilayer film, the heat resistance is improved and in the case the multilayer film produced using such materials is employed (1) for application to various analyses utilizing x-rays and soft x-rays, the heat resistance can be improved as compared with that of a conventional multilayer film type reflecting mirror, so that the reflectance alteration during the use is suppressed as compared with a conventional reflecting mirror and the precision and accuracy can be improved and in the case of using the multilayer film (2) for application to x-ray lithography, the proper exposure duration can precisely be determined owing to the same reason as that for the case of (1) and further (3) the life of the multilayer film itself can be prolonged.
Since a sufficiently high reflectance (50%) of even near 108A wavelength can be obtained by a reflecting mirror of the present invention, the present invention can actualize a light concentrating optical system with high efficiency and a long life especially for a laser plasma x-ray source using a cryotarget. Especially, the present invention can heighten the throughput of a reduction projection lithography to a practically usable level.